1. Field of Invention
The invention relates to a control circuit with anti variations and, in particular, to a control circuit with anti supply voltage and process variations used in the voltage controlled oscillator of electronic circuits and communication systems.
2. Related Art
The voltage controlled oscillator is an important element in a communication system to generate a precise and stable local oscillatory signal for the system to rise, lower, modulate, and demodulate the frequency of a signal. The communication system is evolving toward higher integrity to minimize the system volume and reduce the production cost. Therefore, the voltage controlled oscillator is often integrated with other circuits on a silicon chip.
The local oscillatory signal generated by the voltage controlled oscillator determines the communication quality of the system. When the voltage controlled oscillator is operating with other circuits, the noises at the power supply terminal will enter the voltage controlled oscillator and result in a phase noise, reducing the communication quality of the system. For example, at the receiving terminal of the communication system, an interference signal in the vicinity of the receiving signal will leak into the signal band during the process of lowering and demodulating the frequency at the receiving system. The DC power provided by the battery reduces due to the battery discharge. Solutions in the prior art often employ a voltage stabilizer along with the. voltage controlled oscillator to ensure the normal functioning of the voltage controlled oscillator when the power varies.
On the other hand, the output amplitude of the voltage controlled oscillator varies with the semiconductor process and temperature variations. This phenomenon will affect the operating quality of the communication system. To solve this problem, early technologies amplify the working current to reduce the variations caused by the process and temperature. However, the yield is still limited.
Another method is to use an amplitude control circuit in the voltage controlled oscillator. For example, the voltage oscillator with an amplitude controlled circuit, as disclosed in the U.S. Pat. No. 6,700,450, detects the output amplitude of the voltage controlled oscillator using a peak detector. A voltage signal proportional to the output amplitude is generated to be compared with a reference voltage by a comparator. The comparison result is converted into a current signal to control the current of the voltage controlled oscillator. At the initial state, the voltage detected by the peak detector is lower than the reference signal. After the comparison of the comparator, a current signal is generated to increase the current of the voltage controlled oscillator, enhancing the gain of the transistor in the voltage controlled oscillator. The output amplitude of the voltage controlled oscillator is thus enlarged. When the voltage detected by the peak detector is the same as the reference voltage, the voltage output by the comparator is fixed and the current of the voltage controlled oscillator is also fixed. In this case, the output amplitude of the voltage controlled oscillator reaches a stable value. This amplitude control circuit makes it possible for the output amplitude of the voltage controlled oscillator to be invariant under the semiconductor process and temperature variations.
In the prior art, the supply-voltage stabilization circuit and the amplitude control circuit are independent control circuits. A possible problem, however, is that the common-mode voltage of the voltage controlled oscillator changes when the amplitude control circuit is functioning, interfering and affecting the stability of the supply-voltage stabilization circuit. Although the voltage controlled oscillator in the amplitude control circuit can control its output amplitude within a fixed magnitude, the optimal phase noise condition originally set for the voltage controlled oscillator already changes with the amplitude control. Therefore, the combination of the control circuit and the voltage controlled oscillator increases the complexity of the circuit. The more elements there are, the more noise there is. This is against the design principle of the voltage controlled oscillator, deteriorating the phase noise of the voltage controlled oscillator.